1. Field of the Invention
The present invention relates to radio frequency communications and, more specifically, to an antenna system employed in radio frequency communications.
2. Description of the Prior Art
Radio signals usually start with electrical signals that have been modulated onto a radio frequency carrier wave. The resulting radio signal is transmitted using an antenna. The antenna is a system that generates an electrical field (E field) and a magnetic field (H field) that vary in correspondence with the radio signal, thereby forming radio frequency radiation. At a distance from the antenna, as a result of transmission effects of the medium through which the radio frequency radiation is being transmitted, the E field and the H field fall into phase with each other, thereby generating a Poynting vector, which is given by S=E×H, where S is the Poynting vector, E is the E field vector and H is the H field vector.
Conventional Hertz antenna systems are resonant systems that take the form of wire dipoles or ground plane antennas that run electrically in parallel to the output circuitry of radio frequency transmitters and receivers. Such antenna systems require, for maximum performance, that the length of each wire of the dipole, or the radiator of the ground plane, be one fourth of the wavelength of the radiation being transmitted or received. For example, if the wavelength of the radiation is 1000 ft., the length of the wire must be 250 ft. Thus, the typical wire antenna requires a substantial amount of space as a function of the wavelength being transmitted and received.
A Crossed Field Antenna, as disclosed in U.S. Pat. No. 6,025,813, employs two separate sections which independently develop the E and H fields and are configured to allow combining the E and H fields to generate radio frequency radiation. The result is that the antenna is not a resonant structure, thus a single structure may be used over a wide frequency range. The Crossed Field Antenna is small, relative to wavelength (typically 1% to 3% of wavelength) and provides high efficiency. The Crossed Field Antenna has the disadvantage of requiring a complicated physical structure to develop the E and H fields in separate sections of the antenna. The Crossed Field Antenna also requires an associated complex matching/phasing network to feed the antenna.
Radio Frequency Identification (RF ID) is an emerging field in which a small radio frequency transponder is embedded in or attached to objects so that the objects may be uniquely identified and carry associated information in the memory of the transponder. By international agreement these systems may operate on assigned frequencies from 125 KHz to 4 GHz, with many operating at 13.56 MHz. For practical applications, both the transponder and the associated “reader” of RF ID systems require small antennas, with loop antennas the preferred choice. However, with traditional Hertz loop antennas the distance between the reader and transponder is very limited and the transponder must be parallel to the reader antenna. This is due to low efficiency and narrow bandwidth, and the use of only a magnetic field concentrated around the loop conductor, without the benefit of local radiation. Therefore, there is a need for a compact antenna with high performance.